X-ray crystallographic, energy minimization and quantum mechanical calculations are being employed on compounds of biological interest to give insights into and explanation of their modes of behavior. Various compounds showing promise against cancer and the AIDS virus are being systematically investigated to obtain structural and electronic properties which may help to elucidate the mechanism of their action and thus lead to improved analogs. The x-ray structures of 2',3'-dideoxyinosine (DDI) and Carbovir are being determined. Strain energies and quantum calculations are being carried out on these compounds and on camptothecin.